Recycling and regeneration of lithium-ion battery cathodes

ABSTRACT

A method for regeneration of spent cathode material of lithium-ion batteries involves lithiating the cathode material in a relithiation solution including a reducing agent at a temperature in the range of 60° C. to 180° C. for a sufficient time to heal composition defects in the cathode material. The lithiated material is then sintered to completely recover the properties. The relithiation solution may be a Li-ion source combined with nature-based organic reducing agent such as citric acid, ascorbic acid, tartaric acid, or similar.

RELATED APPLICATIONS

This application claims the benefit of the priority of U.S. ProvisionalApplication No. 63/090,136, filed Oct. 9, 2020, which is incorporatedherein by reference in its entirety.

GOVERNMENT RIGHTS

This invention was made with government support under Grant No.CBET-1805570 awarded by the National Science Foundation. The governmenthas certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to a method for direct regeneration ofspent lithium-ion batteries.

BACKGROUND

Olivine lithium iron phosphate (LiFePO₄ or LFP) is one of the mostwidely used cathode materials for lithium-ion batteries (LIBs) owing toits high thermal stability, long cycle life and low-cost. Theseadvantages have led to the LFP battery share becoming more thanone-third of the entire LIB market, currently dominating applications inpower tools, electric bus, and grid energy storage. The global demand ofLIBs is projected to reach 440 GWh by 2025. This means that millions oftons of spent LIBs will soon to be generated at the ends of theirservice lives (3 to 10 years). Effective recycling and re-manufacturingof spent LIBs can help to reclaim valuable materials, reduce energy usefor mining natural resources, and mitigate environmental pollution fromend-of-life management of waste batteries, making LIBs more affordableand sustainable.

Current efforts on LIB recycling have been focused on recovery ofvaluable metals. For example, pyrometallurgical and hydrometallurgicalprocesses have been used commercially to recycle LIBs containing cobalt(Co) and nickel (Ni). These processes generally involve batterydismantling, smelting and/or acid leaching followed by multi-stepchemical precipitation and separation, in the end breaking LIB cellsdown into simple compounds (e.g., CoSO₄, NiSO₄, Li₂CO₃) that can be usedto re-synthesize new cathode materials. Due to the high value oftransition metals (e.g., —$30/kg for Co), reasonable economic return canbe achieved from such recycling processes, notwithstanding their highoperation cost. Unfortunately, their high energy demand and reliance oncaustic chemicals (acids, oxidation reagents) produce significantgreenhouse gas (GHG) emissions and secondary wastes, raising additionalenvironmental concerns, a frequent criticism heard from individuals whoare resisting the migration to electrical energy sources. Moreover, alarge portion of the cathode's value, represented by their tailoredcomposition and structure, is completely lost through these destructiverecycling processes. Therefore, more efficient approaches withsignificantly reduced energy cost and waste generation are needed,especially for LIBs made without expensive metals, such as LFP, as theeconomic value of their recycled elemental products is insufficient tocompensate for the high cost of pyrometallurgical and hydrometallurgicalprocesses. This is particularly true when considering that world batterymakers have been producing about 100,000 tons/year in total of LFPcathodes since 2015. The large quantity of these batteries that willsoon be retired increases the urgency for better recycling solutions.

Decades of studies have revealed that the performance degradation of LFPcathode is mainly attributable to Li vacancy defects (Li_(v)) and Feoccupation of Li site (Fe_(Li)). The Li_(v) defects not only result inoxidation of Fe to Fe 3+, but also induce partial migration of Fe²⁺ tothe lithium site, forming so-called “anti-site” defects, which block theLi⁺ diffusion pathway. While the charge storage capacity may besignificantly reduced, the morphology and bulk crystal structure ofspent LFP particles often remain unchanged. This failure mechanismprovides a potential opportunity to directly revitalize degraded LFP toform new LFP particles that can be readily used for making new batterycells.

Recycling of spent lithium-ion batteries (LIBs) is an urgent need toaddress their environmental and global sustainability issues. Theinventive method is directed to a solution.

BRIEF SUMMARY

The present invention relates to a method for direct regeneration ofspent LiFePO₄ cathode material of lithium-ion batteries via solutionlithiation under low temperature followed by short sintering. Thisrelatively low energy, mild chemical process enables profitableprocessing even for recycling LiFePO₄ without high-value elements (Ni orCo). The emission of greenhouse gas is demonstrated to be very low. Theeconomical and eco-friendly recycling method shows great potential forapplication in industry.

The inventive method is an efficient and environmentally-benign LIBregeneration method based on defect-targeted healing, which represents aparadigm-shift in LIB recycling strategy. Specifically, by combininglow-temperature aqueous solution relithiation and rapid post-sintering,we demonstrate successful direct regeneration of spent LiFePO₄ (LFP)cathodes, one of the most important materials for EVs and grid storageapplications. The composition, structure, and electrochemicalperformance of LFP cathodes can be revitalized to the same levels as thepristine LFP, even at a wide range of degradation. Life-cycle analysisshows that this defect-targeted direct recycling approach cansignificantly reduce energy usage and greenhouse gas (GHG) emissions,leading to more economic and environmental benefits compared withtoday's hydrometallurgical and pyrometallurgic methods.

In one aspect of the invention, a method for regeneration of spentcathode material of lithium-ion batteries includes: lithiating thecathode material in a relithiation solution comprising at least onereducing agent at a temperature in the range of 60° C. to 180° C. for asufficient time to heal composition defects in the cathode material; andsintering the lithiated material. The relithiation solution may comprisea lithium salt and the at least one reducing agent, wherein the at leastone reducing agent may be one or a combination of nature-derived organicreducing agents. The nature-derived organic reducing agents may beselected from the group consisting of citric acid, ascorbic acid,tartaric acid, oxalic acid, sugars, or a combination thereof. In someembodiments, the relithiation solution may be a mixture of 0.01-4M LiOHsolution and 0.01-2M citric acid. In some embodiments, the lithium saltis selected from the group consisting of LiOH, Li₂SO₄, LiCl, LiC₂H₃O,and LiNO₃. The cathode material may be LiFePO₄. Prior to therelithiating step, the cathode material may be obtained by disassemblingthe lithium-ion battery and removing cathode strips; disposing thecathode strips in a solvent to separate lithium-containing powder fromother components within the cathode strips; and washing and drying theseparated lithium-containing powder. In some embodiments, the sufficienttime is within a range of 1 hour to 18 hours. The temperature may bewithin a range of 60-120° C. and the sufficient time may be at least 5hours. The step of sintering may be performed in a furnace under aninert atmosphere at a sintering temperature in the range of 400° C. to800° C. for a sintering time in a range of 50 to 300 minutes. Thesintering time may include temperature ramping to gradually heat thelithiated material at a controlled rate. The relithiation solution isrecyclable and reusable for subsequent relithiation processes.

In another aspect of the invention, a method for regeneration of LiFePO₄cathode material from a spent lithium-ion battery includes:disassembling the lithium-ion battery and removing cathode strips;soaking the cathode strips in a solvent to separate lithium-containingpowder from other components within the cathode strips; washing anddrying the separated lithium-containing powder; disposing thelithium-containing powder in a vessel with a relithiation solutioncomprising a reducing agent; heating the vessel and solution to atemperature in the range of 60° C. to 180° C. for a sufficient time toheal composition defects in the cathode material; and sintering thelithiated material in an inert atmosphere at a sintering temperature.The relithiation solution may comprise a lithium salt and the at leastone reducing agent, wherein the at least one reducing agent may be oneor a combination of nature-derived organic reducing agents. Thenature-derived organic reducing agents may be selected from the groupconsisting of citric acid, ascorbic acid, tartaric acid, oxalic acid,sugars, or a combination thereof. In some embodiments, the relithiationsolution may be a mixture of 0.01-4M LiOH solution and 0.01-2M citricacid. In some embodiments, the lithium salt is selected from the groupconsisting of LiOH, Li₂SO₄, LiCl, LiC₂H₃O, and LiNO₃.

The sufficient time may be within a range of 1 hour to 18 hours. In someembodiments, the temperature may be within a range of 60-120° C. and thesufficient time is at least 5 hours. The sintering temperature may be inthe range of 400° C. to 800° C., where sintering is performed for asintering time in a range of 50 to 300 minutes. The sintering time mayinclude temperature ramping to gradually heat the lithiated material ata controlled rate. The relithiation solution is recyclable and reusablefor subsequent relithiation processes.

The inventive method employs a green and efficient LIB direct recyclingstrategy based on defect-targeted healing to precisely resolve theLi_(v) and anti-site defects without altering any other properties ofLFP particles. We successfully demonstrate direct regeneration of spentLFP cathodes with various degradation conditions to recover theircomposition, structure, and electrochemical performance to the samelevel as pristine LFP cathode. Unlike pyrometallurgical andhydrometallurgical recycling, such defect-targeted direct recyclingprocess only needs a low concentration of lithium salt, green andlow-cost reducing agent, nitrogen, and water. With proper modification,this method can also be extended to recycle other “low-cost” LIBcathodes such as LiMn₂O₄ (LMO) batteries. Life-cycle analysis of directrecycling of LFP shows that our approach can significantly reduce theenergy usage (by ˜80-90%) and GHG emissions (by ˜75%), leading to moreeconomic and environmental benefits than the current state-of-the-artapproaches.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1E illustrate relithiation kinetics of C-LFP according to theinventive method, where FIG. 1A is a schematic illustration of thesolution relithiation process; FIG. 1B plots the evolution of LFPcomposition during relithiation at different temperatures; FIG. 1C plotsthe saturation vapor pressure of water under various temperaturesassociated with an equipment (schematic) of choice for high pressure (>1bar) and low pressure (<1 bar); FIG. 1D provides XRD patterns of C-LFPand R-LFP with different relithiation times; and FIG. 1E shows thedependence of Li+ apparent diffusion coefficient and required diffusiontime on temperature.

FIGS. 2A-21I illustrate microstructure characterization of different LFPparticles according to an embodiment of the inventive method, whereFIGS. 2A and 2E are STEM images of C-LFP and R-LFP, respectively. FIGS.2B and 2F are STEM images of a C-LFP and a R-LFP particle, respectively;FIGS. 2C and 2G are Fe L-edge EELS spectra of a C-LFP particle and aR-LFP particle, respectively; FIGS. 2D and 2H show Rietveld refinementpatterns of the neutron diffraction data of C-LFP and R-LFP,respectively.

FIGS. 3A-3E illustrate various aspects of the electrochemicalperformance of LFP electrodes, where FIG. 3A is a time-dependent contourplot of diffraction peak intensity in the heating, holding and coolingstages; FIG. 3B shows anti-site defects revolution upon heating,holding, and cooling; FIG. 3C plots the cycling stability of C-LFP,R-LFP, RS-LFP and P-LFP; FIG. 3D compares rate performance of C-LFP,R-LFP, RS-LFP and P-LFP; and FIG. 3E plots long-term cycling stabilityof RS-LFP cycled at 2 C, 5 C and 10 C for 300 cycles.

FIGS. 4A-4C provide XRD patterns and cycling stability, respectively, ofRS-LFP regenerated from C-LFP with different SOHs. C-LFP with 15%, 50%and 60% degradation were regenerated using the same process. From XRDpatterns in FIG. 4A, it can be seen that pure LFP phase was obtainedafter solution relithiation and sintering for all the samples. Thecapacity and stability of LFP with 15% and 60% degradation can berecovered to the same level as the P-LFP, as shown in FIGS. 4B and 4C.The process is schematically illustrated in FIG. 4D showing completerelithiation of C-LPF at different SOHs can be achieved from the samebatch of reaction.

FIGS. 5A-5D provide the results of electrochemical performance whereFIG. 5A plots the results of the charging and discharging process in thefirst cycle at a rate of 0.1C; FIG. 5B plots the cycling stability at arate of 0.5C; FIG. 5C shows the discharge capacity of the full-cell atthe first cycle of 0.1C; and FIG. 5D plots the cycling stability at arate of 0.5C.

FIGS. 6A-6E illustrate economic and environment analyses comparing theinventive approach with other recycling methods, where FIG. 6A providessimplified schematics of pyrometallurgical (“Pyro”) andhydrometallurgical (“Hydro”) and direct recycling (“Direct”) methods, aswell as cathode production from virgin materials mining; FIGS. 6B and 6Cshow total energy consumption and GHG emissions per kg of recycled cellfrom pyrometallurgical, hydrometallurgical and direct recycling,respectively; and FIGS. 6D and 6E compare total energy consumption andGHG emissions per kg of cathode production from virgin materials andspent batteries using direct recycling process.

FIGS. 7A and 7B are XRD patterns and cycling performance, respectively,of the C-LFP and RS-LFP regenerated with a fresh and recycledrelithiation solution of LiOH and CA.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

To demonstrate the inventive defect-targeted direct recycling method,commercial LFP cells were cycled for up to 6500 cycles in the 2.5-3.8 Vvoltage range to reach a capacity decay of up to 50%. The cells weredisassembled and LFP powders were harvested from the cathodes followingthe procedures described by Y. Shi, et al., ((2018), “Effectiveregeneration of LiCoO2 from spent lithium-ion batteries: A directapproach towards high-performance active particles. Green Chem. 20,851-862.) The collected cycled LFP particles (denoted as “C-LFP”) weresubject to relithiation treatment in a Li-containing aqueous solutionwith controlled temperature and time. The relithiated LFP powders(R-LFP) were washed thoroughly using deionized (DI) water, dried, andthen subjected to post sintering to complete the entire regenerationprocess.

LiFePO₄, “LFP”, cells were cycled in the voltage range of 2.5-3.8 Vusing an Arbin battery tester for over 6500 cycles and then dischargedto 2 V at C/10 (1C=170 mA g⁻¹) before disassembly. The cathode stripswere thoroughly rinsed with dimethyl carbonate (DMC) to remove residualelectrolyte. After drying, the cathode strips were soaked in NMP(N-Methylpyrrolidone) or other appropriate solvent for 30 min followedby sonication for 20 min, which removed the LFP powders, binder, andcarbon black from the aluminum substrates. The obtained suspension wascentrifuged at 3500 rpm for 5 min and the cycled LFP (C-LFP) powderswere precipitated, separated, and dried for regeneration.

Fresh cells were directly discharged to 2 V at C/10 without any cyclingbefore disassembly, and the harvested LFP material served as thereference material for comparison.

The C-LFP was regenerated through a solution relithiation followed by ashort annealing process. For the solution relithiation treatment, LFPpowders harvested from cycled cells were loaded into a 100 mL reactorfilled with 80 mL of 0.2 M LiOH and 0.08 M CA solution. An exemplaryrange for the solution composition will be 0.01-4M Li solution and0.01-2M reducing agent. The reactor was kept at a wide range oftemperatures for various operation times for relithiation. Therelithiated LFP (R-LFP) powders were washed thoroughly with deionizedwater, and dried. The R-LFP was then mixed with excess 4% Li₂CO₃ andsubject to thermal sintering at different temperatures for 2 h in aninert (nitrogen) atmosphere with a temperature ramping rate of 5° C.min⁻¹. The final recycled LFP is denoted as RS-LFP. It should be notedthat while the experiments described herein use LiOH as the source oflithium ions, as will be apparent to those of skill in the art, otherlithium ion sources, including lithium salts such as Li₂SO₄, LiCl,LiC₂H₃O₂, LiNO₃, among others, may be used.

The crystal structure of the powders was examined by X-ray powderdiffraction (XRD) employing Cu Kα radiation. The X-ray photoelectronspectroscopy (XPS) measurement was performed with Kratos AXIS Ultra DLDwith Al Kα radiation. The composition of pristine, degraded, andregenerated LFP cathode was measured by an inductively coupled plasmamass spectrometry (ICP-MS). HRTEM was recorded on JEOL-2800 at 200 kVwith Gatan OneView Camera. STEM-EDS was performed on primary particles aat annular dark field (ADF) mode using the same instrument. STEM-EELSwas performed on JEOL JEM-ARM300CF at 300 kV, equipped with doublecorrectors. Ex-situ neutron diffraction patterns were collected in thehigh-resolution mode (Δd/d ˜0.25%) for a duration of 2 h under thenominal 1.1 MW SNS operation, and then processed using VDRIVE software,a suite of neutron diffraction data reduction and analysis softwareavailable from Oak Ridge National Laboratory. Operando neutrondiffraction data were collected in the high intensity mode (Δd/d ˜0.45%)while the powders were heated and cooled in a furnace under nitrogenatmosphere.

To evaluate electrochemical performance using electrodes with moderatemass loading, different LFP powder sample was mixed with polyvinylidenefluoride (PVDF) and Super P65 in NMP at a mass ratio of 8:1:1. Theresulted slurries were cast on aluminum foils followed by vacuum dryingat 120° C. for 6 h. Circle-shape electrodes were cut and compressed,with controlled active mass loading of about 3-5 mg cm′. To makeelectrodes for high-mass loading half-cells and pouch full-cellstesting, the cathode casting was made with a commercial relevant ratio(RS-LFP: Super P: PVDF=95:2:3) and the mass loading of active materialwas controlled at −19 mg/cm 2. Galvanostatic charge-discharge wascarried out in the potential range of 2.5-3.8 V with the assembledcells. The electrolyte was LP40 (1M LiPF₆ in ethylene carbonate/diethylcarbonate=50:50 (v/v)). The cells were cycled with activation for 3cycles at 0.1C followed by extended cycling at higher rates. Theelectrochemical impedance spectroscopy (EIS) tests were performed atdischarged state in the frequency range of 10⁶ Hz to 10⁻³ Hz with signalamplitude of 10 mV by a Metrohm Autolab potentiostat.

The key to regenerate C-LPF is to precisely resolve the Li_(v) andanti-site defects. A high activation energy (1.4 eV) is required for Feions to migrate back to the original position (M2) because of the strongelectrostatic repulsion of high valence state of Fe³⁺ during migration.Referring to FIG. 1A, the positions of Li and Fe in a perfect olivinestructure are defined as M1 and M2 sites, respectively. The upper panelof the figure shows C-LFP with Li vacancies (Li_(v)) and Fe occupationin Li site (Fe_(Li)), while the lower panel shows R-LFP with all theFe³⁺ being reduced to Fe²⁺ through processing via CA (citric acid,center panel) in a LiOH solution. Park et al. (“Anti-site reordering inLiFePO₄: Defect annihilation on charge carrier injection”, Chem. Mater.26, 5345-5351 (2014)), demonstrated successful anti-site re-ordering viadeep discharging to 1.5V (vs. Li/Li′) at an extremely low rate of C/100(“1C” represents charge or discharge in one hour). The theoretical studyalso reveals that a reductive environment can lower the activationbarrier which in turn can facilitate Fe migration. Therefore, foreffective direct regeneration of C-LFP, the most critical step is toreduce Fe′ and re-dose lithium ions (Lit) into C-LFP.

The half electrode potential of LFP electrode is 0.40 V (vs. standardhydrogen electrode or SHE) (Equation 1).

FePO₄+Li⁺ +e ⁻ →LiFePO ₄ E(FePO₄/LiFePO₄)=0.40 V  (1)

1/2C₆H₈O₇ −e ⁻→1/2C₅H₆O₅+1/2CO₂+H⁺E(C₅H₆O₅/C₆H₈O₇)=−0.34 V  (2)

FePO₄+Li⁺+1/2C₆H₈O₇ →LiFePO₄+1/2C₅H₆O₅+1/2CO₂+H⁺E(FePO₄/C₆H₈O₇)=−0.74V  (3)

A variety of reducing agents may be used to proceed reduction of Fe³⁺.Inorganic reductants such as NaBH₄, Na₂S₂O₃, and hydrogen peroxide(H₂O₂) are well known for their effectiveness as reducing agents invarious combinations. Nature-derived organic reductants are particularlyinteresting for the inventive process as they are safe andenvironmentally benign. Examples of appropriate reductants includecitric acid (C₆H₈O₇), oxalic acid (C₂H₂O₄), ascorbic acid (C₆H₈O₆),tartaric acid (C₄H₆O₆), which may be used alone or in combinations.Glucose (C₆H₁₂O₆) and other sugars are also possible nature-derivedorganic reductants that may be employed. For example, citric acid (CA),concentrated in citrus fruits, has a redox potential of ˜−0.34 V (vs.SHE) (Equation 2), which can be an ideal candidate to assist thereduction of C-LFP. The Gibbs free energy for equation (3), the completereaction by combining (1) and (2), is calculated to be −56.35 kJ/mol(see details in Supporting Information), indicates that the relithiationreaction of degraded LFP is thermodynamically favorable. In ourexperiment design, CA in the Li-containing aqueous solution donateselectrons to reduce Fe³⁺, reducing electrostatic repulsion andsubsequently lowering the migration barrier to move Fe²⁺ from the M1site back to the M2 site, which facilitates the solution Li⁺ diffusioninto the Li-deficient C-LFP particles.

The evolution of LFP composition during the solution relithiation wasmonitored by inductively coupled plasma mass spectrometry (ICP-MS). Wefirst tested relithiation at 180° C., which is the minimum temperaturerequired for relithiation of degraded layered oxides such as LiCoO₂ andLiNi_(1-x-y)Co_(x)Mn_(y)O₂ cathodes. As shown in FIG. 1B, the Licomposition of the C-LFP particles increased from 0.5 to 1.0 as therelithiation time was extended to 5 hr. Note that an autoclave reactorthat can hold pressure greater than 11 bar (saturation pressure ofwater) is preferably used for this operation (FIG. 1C). Effectiverelithiation at sub-boiling temperature allows pressurized reactors tobe replaced by low-cost vessels without extra safety precautions.Composition analysis of the relithiation solution before and afterreaction showed that 1.9 mol % of Fe was leached from the initial LFP.This may be attributed to the trace amount of Fe₂O₃ generated in thedegraded LFP after long-term cycling (FIG. 1D). From the quality controlperspective, leaching the residual Fe₂O₃ phase might be desirable as itprovides high purity LFP phase in the regenerated product. The gradualdiminishment of FePO₄ peaks (marked by the dashed gray vertical lines)indicates the conversion of the FePO₄ phase to LFP phase.

With the goal of minimizing the energy consumption for the process,lower temperatures were explored. Surprisingly, as shown in FIG. 1B,reducing the solution temperature to as low as 80° C. resulted in anegligible change in the relithiation kinetics. Further extending thetreatment time allowed continuous reduction of the solution temperaturefor relithiation. For example, 100% composition recovery can be achievedat a temperature of 70° C. and 60° C. after 10 and 17 hours ofrelithiation, respectively.

The Li⁺ apparent diffusion coefficient and time at differenttemperatures were calculated. The details of the calculation are shownas the following:

$D_{Li^{+}}^{app} = \frac{R^{2}T^{2}}{2A^{2}n^{4}F^{4}C^{2}\sigma^{2}}$

where R is the gas constant, T the absolute temperature, A the interfacebetween the cathode and electrolyte (A=1.6 cm²), n the number ofelectrons involved in the reaction, F the Faraday constant, C theconcentration of Li⁺ in the electrode (=ρ/M) based on the molecularweight of LFP (M) and density (ρ), and a the Warburg factor. The Warburgfactor can be obtained from the slope of Z′ vs. ω^(−1/2) plots (co isthe angular frequency) in the Warburg region:

Z _(real)=σω^(−1/2)

Based on the obtained slope, the Li⁺ apparent diffusion coefficient forthe LFP sample was calculated to be 1.05×10⁻¹⁵ cm²/s.

The apparent diffusion coefficient in solids at different temperaturescan be predicted by the Arrhenius equation.

D _(Li) ₊ ^(app) =D ₀ e ^(−E) ^(a) ^(/kT)

where D_(Li) ₊ ^(app) is the lithium apparent diffusion coefficient,E_(a) the activation energy (3.1 eV), k the Boltzmann constant(8.617×10⁻⁵ eV/K), and Do the pre-exponential factor.

The relation between the mean diffusion time of Li⁺ and the D_(Li) ₊^(app) diffusion coefficient can be estimated using the following theequation:

$t = \frac{R^{2}}{4D_{Li^{+}}^{app}}$

where t is the Li⁺ diffusion time, D_(Li) ₊ ^(app) the diffusioncoefficient at different temperature, and R the diffusion length (˜100nm). The calculated diffusion time as a function of temperature isplotted in FIG. 1E.

Electrochemical impedance spectroscopy (EIS) measurement showed a Li⁺apparent diffusion coefficient (D_(Li) ₊ ^(app)) of 1.05×10⁻¹⁵ cm²/secfor Li_(0.5)FePO₄, which is consistent with previous reports. Assumingan average LFP particle size of 100 nm and using the above D_(Li+), thecalculated Li⁺ diffusion time matches well with the relithiation time inthe experiment, demonstrating the solution relithiation kineticsgenerally follows the semi-infinite solid-state diffusion mechanism. Theeffective relithiation at temperatures below the boiling point of waterallows the process to be conducted at ambient pressure. This allowspressurized reactors to be replaced by low-cost vessels that do notrequire extra safety precautions, making the process even more practicalfor large scale operation.

To further validate the critical role of citric acid (CA), the sameC-LFP was treated with a LiOH solution without CA. As expected,continuous oxidation of (LiFePO₄) to Fe₂O₃ and Fe₃O₄ was observed. Thisresult also confirms the effectiveness of defect-targeted healingenabled by CA. In addition, CA is a widely used low-cost (˜0.55 $/kg)additive in food industry, and it only generates CO₂, H₂O andacetonedicarboxylic acid (C₅H₆O₅, ˜10 $/kg) during the relithiationprocess. It should be also noted that C₅H₆O₅, an important intermediatefor drug synthesis, is traditionally prepared by decarbonylation of CAin fuming sulfuric acid. This suggests that our direct LFP recyclingprocess may be coupled with suitable precursors to offer an alternativeroute for green synthesis of valuable organic molecules. Other reducingagents such as ascorbic acid (“AA”) (E=−0.55 V)²³ and tartaric acid(“TA”) (E=−0.23 V) have demonstrated similar functionality to regenerateC-LFP, offering a variety of options for low-cost reducing agents.

X-ray diffraction (XRD) patterns of the C-LFP and samples after solutionrelithiation for different durations (denoted as “R-LFP”) furtherillustrate the phase transition of degraded LFP during the solutionrelithiation process. For example, referring to FIG. 1D, the C-LFP showsintense peaks at 2θ=18° and 32° (highlighted by the dashed gray lines),which are attributed to the existence of the FePO₄ phase due to lithiumloss. As the relithiation time increased from 1 hr to 5 hr at atemperature of the intensities of these peaks gradually diminished andthen disappeared, suggesting the conversion of the FePO₄ phase to theLFP phase.

High-angle annular dark-field (HAADF) scanning transmission electronmicroscopic (STEM) images were obtained to further understand therelithiation mechanism at the atomic level. For LFP cathode after over6500 cycles, the particles still show well-defined crystallinity with aconformal carbon coating (2-3 nm) retained on the surface (FIG. 2A). Theelectron energy loss spectroscopy (EELS) experiment was carried out toprobe the valence states of O and Fe from the surface to the inner sideof the particles (FIG. 2B). From one representative particle, the OK-edge and Fe L-edge spectra from the surface (point 1) to the innerside (point 6) of the C-LFP particle were compared. For the C-LFP, the Opre-peak gradually emerged from the surface to the bulk, suggesting thepresence of the Fe′ inside the C-LFP particle. The Fe L-edge graduallyshifted from 707.93 eV to 709.65 eV, as shown in FIG. 2C and Table 1below, suggesting the dominant presence of Fe′ in the bulk. Point 1indicates the surface of the particle and point 6 indicates the innerside of the particle.

TABLE 1 C-LFP RS- LFP Fe L edge Position (eV) Fe L edge Position (eV) 1(surface) 707.93 1 (surface) 707.75 2 708.27 2 707.23 3 708.71 3 707.484 709.26 4 707.61 5 709.46 5 707.72 6 (bulk) 709.65 6 (bulk) 707.81

In EELS spectra taken from another representative particle, a clear OK-edge pre-peak showed up in the spectrum obtained from the particlesurface, indicating the presence of Fe³⁺ on the surface. The above EELSresults demonstrate the coexistence of FePO₄ and LiFePO₄ phases andtheir random distribution in different particles. Although severaltwo-phase models have been proposed to understand the local structure ofdelithiated LFP, including the shrinking-core model, mosaic model, anddomino-cascade model, they are mainly established upon the first chargeand discharge cycle. Our results suggest a high inhomogeneity of phasedistributions for the LFP particles after long-term charge/dischargecycles.

FIG. 2D shows the Rietveld refinement pattern of the neutron diffractiondata of the C-LFP with the detailed structural information listed inTable 2 below, where Phase 1 LiFePO₄: Space group: Pnma, R_(wp)=2.56%,a=10.2926(10) Å, b=5.9905(6) Å, c=4.6989(4) Å, α=β=γ=90°, Fraction:52.9%, Phase 2 FePO₄: Space group: Pnma, R_(wp)=2.56%, a=9.8284(9) Å,b=5.7955(5) Å, c=4.7831(4) Å, α=β=γ=90°, Fraction: 47.1%.

TABLE 2 52.9% Phase 1 LiFePO₄ 47.1% Phase21 FePO₄ Atom Site Wyckoffpositions Occupancy Site Wyckoff positions Occupancy Li 4a 0 0 00.952(17) NA Fe 4a 0 0 0 0.048(17) 4a 0 0 0 0.048(7) Fe 4c 0.2831(4)0.25 0.9791(10) 0.952(17) 4c 0.2741(4) 0.25 0.9524(8) 0.952(17) Li 4c0.2831(4) 0.25 0.9791(10) 0.048(17) NA P 4c 0.0985(7) 0.25 0.4199(14) 14c 0.0909(7) 0.25 0.4066(15) 1 O 4c 0.0952(7) 0.25 0.7464(19) 1 4c0.1186(6) 0.25 0.7176(14) 1 O 4c 0.4570(7) 0.25 0.1924(15) 1 4c0.4435(7) 0.25 0.1583(13) 1 O 8d 0.1685(5) 0.0521(8) 0.2809(11) 1 8d0.1690(5) 0.0467(3) 0.2516(11) 1

Overall, the C-LFP exhibits 47.1% of Li deficiencies (loss) and 4.81%Fe/Li anti-site defects. The computational study by Malik et al.(“Particle size dependence of the ionic diffusivity”, Nano Lett. (2010),10, 4123-4127) showed that 0.1% anti-site can cause ˜5% of Li⁺ to betrapped in the defects in a 100 nm LiFePO₄ particle. Generally, Liinventory loss is considered to be the main reason for capacitydegradation of LFP batteries while the impact of anti-site defects wasoften overlooked. Olivine LFP has Pnma space group with Li⁺ confined inchannels propped up by the interconnecting FeO₆ octahedra and PO₄tetrahedra. Since the direction is the exclusive pathway for Li⁺diffusion, such a significant occupation of Fe²⁺ in the Li sites canblock Li⁺ diffusion, which leads to loss of capacity and rateperformance.

For the R-LFP sample, all the Fe²⁺ around the Li⁺ show ordered structurealong the [010] direction, as revealed by the HAADF-STEM image of arepresentative R-LFP particle. (FIG. 2E). Continuous Li⁺ diffusionchannels along the direction is shown. The carbon shell was alsoretained after solution relithiation. The disappearance of the Opre-peak and the unchanged Fe L-edge at 707 eV (FIG. 2F) confirmed thatthe oxidation state of Fe from the surface to the bulk remained as 2+.The neutron diffraction data (FIG. 2G) further confirms that a pureorthorhombic LFP phase was obtained after the relithiation treatment,with the ratio of anti-site defects reduced to as low as 2.2%, which iseven lower than the pristine LFP (denoted as “P-LFP”) (2.5%).

FIG. 211 provides the Rietveld refinement pattern of the neutrondiffraction data of R-LFP. Table 3 below lists the structural parametersobtained from Rietveld refinement of neutron diffraction pattern ofR-LFP, where LiFePO₄: Space group: Pnma, R_(wp)=4.09%, a=10.3146 (9) Å,b=6.0000 (6) Å, c=4.6909 (4) Å, α=β=γ=90°.

TABLE 3 Atoms Site Wyckoff positions Occupancy Li 4a 0 0 0 0.978(7) Fe4a 0 0 0 0.022(7) Fe 4c 0.2809(3) 0.25 0.9741(9) 0.978(7) Li 4c0.2809(3) 0.25 0.9741(9) 0.022(7) P 4c 0.0965(6) 0.25 0.4139(11) 1 O 4c0.0960(7) 0.25 0.7475(16) 1 O 4c 0.4556(5) 0.25 0.1983(12) 1 O 8d0.1636(4) 0.0525(6) 0.2774(7) 1

High crystallinity LFP (denoted as “RS-LFP”) with further reducedanti-site defects were obtained after a short sintering treatment of theR-LFP. The XRD patterns for each sample were examined to identifypossible structural changes. While the average grain size grew from 72to 96 nm as the sintering temperature increased from 400 to 800° C., nophase changes were observed. A uniform carbon coating also remained onthe particle surface, as indicated by a homogenous distribution of C, Pand Fe elements in element mapping, further suggesting targeted healingof the composition and microstructure defects in C-LFP.

Operando neutron diffraction was performed to quantify the evolution ofFeu anti-site defects during sintering. The time-dependent contour plotof peak intensity shown in FIG. 3A further confirms that pure LFP phasewas maintained during heating and cooling, demonstrating good stabilityof the R-LFP particles. After cooling, the ratio of anti-site defectswas reduced to 1.5%, which is further reduced compared with that of theP-LFP (2.5%). In FIG. 3B, the dots with error bars correspond to theratio of the anti-site defects. By contrast, significant phaseimpurities (e.g., Li₃PO₄, Fe₂P) always exist after the solid-statereaction-based regeneration process (SS), where Li-containing precursor(e.g., Li₂CO₃, LiOH) was mixed with degraded LFP particles forsintering. It is an added benefit of the solution-based relithiationprocess to ensure homogenous Li distribution inside LFP particles,acting to eliminate phase impurities after sintering.

Electrochemical performance of the LFP samples was first evaluated usinghalf cells. The cycling test started with 0.1C (1C=170 mA g⁻¹)activation for 2 cycles followed by 0.5C for another 100 cycles (FIG.3C). The P-LFP showed a capacity of 161 mAh g⁻¹ at 0.5C with negligiblecapacity decay after 100 cycles. The capacity of the C-LFP recoveredfrom spent cells was only 103 mAh g⁻¹ at 0.5C, which further decreasedto 98 mAh g⁻¹ after 100 cycles. The initial capacity of the R-LFP wasdramatically improved to 159 mAh g⁻¹, indicating the restoredelectrochemical activity after solution relithiation. However, only93.7% of the initial capacity was maintained after 100 cycles. Such adegradation is probably associated with the Li⁺/proton exchange duringthe aqueous relithiation, which has a negligible effect on the crystalstructure, but induced side reactions due to presence of protons. Thefollowing short sintering step helps to create more stable particlesthat can deliver the same capacity and stability as the P-LFP. It wasfound that temperatures that were too low might aggravate structuraldefects, as has been reported in earlier studies of LFP synthesis, whiletemperatures that were too high could lead to larger grain size. Bothcases led to inferior capacity of RS-LFP to P-LFP. When the sinteringtemperature increased from 400 to 600° C., the discharge capacity (at0.5 C) of the first cycle increased from 148 to 159 mAh g⁻¹. When thetemperature further increased from 600° C. to 800° C. the capacitydropped to 141 mAh g⁻¹. The capacity retention was 99%, 99%, 99%, 97%and 94% for the samples sintered at 400, 500, 600, 700 and 800° C.,respectively.

Thermal sintering at 600° C. for 2 hours enables RS-LFP to deliver acapacity of 159 mAh g⁻¹ at 0.5 C with less than 1% of capacity lossafter 100 cycles. With extended cycling at 0.5 C for 1000 cycles, theRS-LFP can still deliver a capacity of 150 mAh/g.

The rate capability of the C-LFP can be also recovered after thecomplete regeneration, as shown in FIG. 3D. Specifically, the P-LFP candeliver a capacity of 163, 141 and 99 mAh g⁻¹ at 0.2, 2 and 10C,respectively. The capacity of RS-LFP increased to 162, 144 and 102 mAhg⁻¹, superior to that of P-LFP, especially at high rates. By contrast,the C-LFP can only provide a capacity of 115, 82 and 66 mAh g⁻¹, due tothe Li loss and structure defects. In addition, the regenerated LFP alsoexhibited excellent long-term cycling stability. No obvious capacityloss was observed after 300 cycles at 2C, 5C and 10C rates (FIG. 3E).The significantly improved rate performance and high stability of theRS-LFP suggest that both the composition and structure of C-LFP havebeen completely recovered after the relithiation and short sinteringtreatment.

The high loading half-cell showed an initial capacity of 156 mAh/g andmaintained at 157 mAh/g after 50 cycles at 0.5C. The pouch cell (3 cm×3cm) can deliver a capacity of 28.6 mAh (3.17 mAh/cm 2) at a rate of 0.1Cand without capacity degradation after cycling for 30 cycles. Theseresults further suggest the significant potential of using directlyregenerated LFP to manufacture new cells without sacrificing cell-levelperformance.

In general, 20% capacity loss is considered to be the end of life forelectric vehicle (EV) batteries. By considering secondary use, one canassume that 50% capacity decay might be the lower limit of the servicelife of a LIB for any applications. In reality, a LIB waste stream mightconsist of cells with various degradation conditions. Therefore, wetested our method on a mixture of cycled cathode materials withstates-of-health (SOH) of 40%, 50% and 85% to fully examine theeffectiveness of our process. Subjected to the same regeneration processas described earlier, the cycled LFP mixture showed complete recovery ofcomposition, structure, and electrochemical performance to the samelevel as P-LFP Using the same process, RS-LFP was regenerated from C-LFPwith different SOHs: 15%, 50% and 60% degradation. From the XRD patternsin FIG. 4A, it can be seen that pure LFP phase was obtained aftersolution relithiation. FIGS. 4B-4C provide the capacity and stability ofLFP with 60% degradation, showing recovery to the same level as theP-LFP. The process is schematically illustrated in FIG. 4D showing thatcomplete relithiation of C-LPF at different SOHs can be achieved fromthe same batch of reaction.

These results suggest significant advantages of using thelow-temperature solution relithiation method to treat spent batterieswith a diverse range of health conditions, as the cathodes all reach astoichiometric composition due to self-saturation.

In order to further examine the practical applications of theregenerated LFP (RS-LFP), commercial relevant thick electrodes with amass loading of −19 mg/cm 2 were prepared. These were then used toassemble both half cells (with Li metal as the counter electrode) andpouch cells (with graphite as the anode).

The corresponding electrochemical performance was evaluated byconstructing a cathode casting with a commercial relevant ratio (RS-LFP:Super P: PVDF=95:2:3) and the mass loading of active material was ˜19mg/cm². The electrolyte was LP40 (1M LiPF₆ in EC/DEC) and the cells werecycled with activation for 3 cycles at 0.1C and followed by extendedcycling at 0.5C.

FIG. 5A plots the results of the charging and discharging process in thefirst cycle at a rate of 0.1C, yielding a reasonable first cycleColumbic Efficiency (97.6%) with a capacity of 166 mAh/g. The half-celldelivers capacities of 169 and 165 mAh/g. The cycling stability was nextevaluated at a rate of 0.5C, with the results plotted in FIG. 5B. Forsuch a high mass loading cathode, the RS-LFP delivered a capacity of 156mAh/g initially and maintained a capacity of 157 mAh/g after 50 cycles,suggesting good stability. FIG. 5C shows that the discharge capacity ofthe full-cell at the first cycle of is 166 mAh/g, almost achieving thetheoretical capacity (172 mAh/g) of LFP. It should be noted that theassembled pouch cell can reach a total capacity of 28.6 mAh (3.17 mAh/cm2). FIG. 5D plots the cycling stability at a rate of 0.5C. Notably,these results are comparable to several of the leading commercial LFPsuppliers around the world: A123 Systems, LLC (154 mAh/g), PhostechLithium Inc. (156 mAh/g), Likai (158 mAh/g), Valence Technology, Inc.(149 mAh/g), and Sitelan (156 mAh/g). The capacity can still retain 157mAh/g after 30 cycles. These results further confirm that theregenerated LFP exhibited excellent electrochemical performance even incommercially-relevant thick electrodes, showing great potential forpractical application.

FIG. 6A provides a brief flowchart of cathode regeneration from directrecycling of cathodes from spent LIB materials, as well aspyrometallurgical recycling (“Pyro”), hydrometallurgical recycling(“Hydro”), and virgin cathode material production. It should be notedthat currently, pyrometallurgical and hydrometallurgical recyclingroutes are not used commercially to recover cathode material from spentLFP batteries due to economic loss. They are included here as potentialend-of-life management options for LFP batteries, on the assumption thatbattery recycling will be mandated before new recycling technologiesbecome available. Compared with other processes, the clear advantages ofdeveloping the direct recycling process for LFP lie in: 1) simplifiedoperation facilities and processes, 2) reduced operation temperature andtime, and 3) eliminating the usage of strong acid and base. Thesefeatures are associated with the potential economic and environmentalbenefits that can be analyzed by the EverBatt model developed by ArgonneNational.

The three different recycling methods are modeled assuming 10,000 tonsof spent batteries annual plant processing capacity (FIG. 6B). Thelife-cycle total energy use for pyrometallurgical and hydrometallurgicalprocesses are 18.4 and 30.6 MJ LFP cell, respectively. In thepyrometallurgical process, 55% of the energy use is attributed to hightemperature smelting. In the hydrometallurgical process, 87.8% of theenergy use comes from upstream production of the chemicals consumed inthe process. The total energy use for direct recycling is only 3.5 MJLFP cell, significantly lower than the other processes. GHG emissionsare also an important factor to consider when evaluating a recyclingapproach. As shown in FIG. 6C, the total GHG emissions released from thedirect recycling process are only 26.6% and 27.7% of those frompyrometallurgical and hydrometallurgical processes, respectively.Moreover, the total energy use per kg of cathode made from directrecycling of the spent batteries is only 22.3% of that for cathodeproduced from virgin materials (FIG. 6D). The GHG emissions from cathodeproduction via direct regeneration of spent batteries is 46.2% lowerthan that from virgin materials (FIG. 6E).

The total cost of pyrometallurgical, hydrometallurgical, and directrecycling is $3.4, $2.4 and $2.1 per kg of spent battery cellsprocessed, respectively. It should be noted that any recycled Al, Cu,graphite is assumed sold to recover some cost, but the net revenuecannot cover the high cost of the pyrometallurgical andhydrometallurgical recycling processes due to the use of expensiveequipment, significant quantities of materials, and high energyconsumption. This is the main reason why the industry currently does notcycle LFP cells. In contrast, using direct recycling as describedherein, the regenerated cathode materials can be used by cellmanufacturers without further re-synthesis, resulting in a potentialprofit of 1.04 $ per kg of recycled spent batteries.

The significant reductions in total energy use, GHG emissions, and lowercost afforded by the inventive the low-temperature aqueousrelithiation-based direct regeneration method provide an importantoption for spent LIB recycling. Existing methods for LFP recyclingcontinue to be based on hydrometallurgical processes or otherdestructive processes. Ideally, solid-state sintering by adding adesired amount of lithium back into spent LFP cathode powders may alsorestore their original composition. However, it may be practicallychallenging to determine an accurate quantity of lithium dosage for alarge number of spent cells having significantly different SOHs. Moreimportantly, defect-targeted healing cannot be achieved as manifested bythe relatively low capacity of recycled LFP from solid-state sintering.While chemical lithiation in an aprotic solvent (e.g., acetonitrile)using strong reducing agent may also be used to re-functionalize spentLFP, the highly caustic nature of such system may restrict its practicalapplication.

Another advantage of using the inventive ambient condition solutionprocess is that the relithiation solution itself can be also recycled.For example, the used solution with LiOH and CA was tested to relithiatea second batch of spent LFP under the same conditions. The XRD patternsand cycling stability of RS-LFP regenerated with a fresh and recycledsolution are compared in FIGS. 7A and 7B. As seen in in FIG. 7B, thefresh and recycled lines completely overlap. Thus, a pure LFP phase canbe obtained even using the recycled solution of LiOH and CA. Thecapacity and stability of RS-LFP reached the same level as thatregenerated with a fresh solution of LiOH and CA. The successfuldemonstration of recycling and reuse of the relithiation solution ofLiOH and CA adds further efficiencies to reduce the overall operationalcosts of the inventive direct recycling method.

The methods and procedures described herein demonstrate adefect-targeted healing method for more efficient and sustainablerecycling of spent LIB materials. These improvements represent aparadigm-shift towards potentially profitable and green recycling ofLIBs that are simply not possible with existing recycling processes. Thecomplete recovery of the electrochemical performance of spent LFPcathode to the level of pristine material can improve the marketacceptance of recycled battery materials. Moreover, under the Everbattmodel, assuming 10,000 tons of annual plant processing capacity of spentbatteries suggests that our direct regeneration route has low energyconsumption of 3.5 MJ LFP cell (accounting for only 19% and 11% of pyro-and hydrometallurgical processes, respectively) and low GHG emissions of0.7 kg/kg LFP cell (26.6% and 27.7% of pyro- and hydrometallurgicalprocesses, respectively). Importantly, the cost of direct regenerationcan be reduced to $2.1 per kg spent LFP cell, compared with $3.4 and$2.4 for pyro- and hydrometallurgical processes, respectively. It shouldbe noted that even though remain some uncertainties in terms of thecosts of battery collection and transportation, they can be expected tobe the same regardless of which recycling process is used since theywill presumably be collected from the same source. Thus, theefficiencies obtained through direct recycling are attributable to thesignificantly improved operation design and reduced chemical usage.

1. A method for regeneration of spent cathode material of lithium-ionbatteries, the method comprising: lithiating the cathode material in arelithiation solution comprising at least one reducing agent at atemperature in the range of 60° C. to 180° C. for a sufficient time toheal composition defects in the cathode material, wherein the at leastone reducing agent is a nature-derived organic reducing agent selectedfrom the group consisting of citric acid, ascorbic acid, tartaric acid,oxalic acid, sugars, or a combination thereof; and sintering thelithiated material.
 2. The method of claim 1, wherein the relithiationsolution comprises a lithium salt.
 3. (canceled)
 4. The method of claim1, wherein the nature-derived organic reducing agent comprises citricacid and the relithiation solution comprises 0.01-4M LiOH and citricacid.
 5. The method of claim 2, wherein the lithium salt is selectedfrom the group consisting of LiOH, Li₂SO₄, LiCl, LiC₂H₃O₂, and LiNO₃. 6.The method of claim 1, wherein the cathode material is LiFePO₄.
 7. Themethod of claim 1, wherein the cathode material is obtained prior tolithiating by: disassembling the lithium-ion battery and removingcathode strips; disposing the cathode strips in a solvent to separatelithium-containing powder from other components within the cathodestrips; and washing and drying the separated lithium-containing powder.8. The method of claim 1, wherein the sufficient time is within a rangeof 1 hour to 18 hours.
 9. The method of claim 1, wherein the temperatureis in the range of 60-120° C. and the sufficient time is at least 5hours.
 10. The method of claim 1, wherein sintering is performed in afurnace under an inert atmosphere at a sintering temperature in therange of 400° C. to 800° C. for a sintering time in a range of 50 to 300minutes.
 11. The method of claim 10, wherein the sintering timecomprises temperature ramping to gradually heat the lithiated materialat a controlled rate.
 12. The method of claim 1, wherein therelithiation solution is recyclable and reusable for subsequentrelithiation processes.
 13. A method for regeneration of LiFePO₄ cathodematerial from spent lithium-ion batteries, the method comprising:disassembling the lithium-ion battery and removing cathode strips;soaking the cathode strips in a solvent to separate lithium-containingpowder from other components within the cathode strips; and washing anddrying the separated lithium-containing powder; disposing thelithium-containing powder in a vessel with a relithiation solutioncomprising at least one reducing agent, wherein the at least onereducing agent is a nature-derived organic reducing agent is selectedfrom the group consisting of citric acid, ascorbic acid, tartaric acid,oxalic acid, sugars, or a combination thereof; heating the vessel andsolution to a temperature in the range of 60° C. to 180° C. for asufficient time to heal composition defects in the cathode material; andsintering the lithiated material in an inert atmosphere at a sinteringtemperature.
 14. The method of claim 13, wherein the relithiationsolution comprises a lithium salt.
 15. (canceled)
 16. The method ofclaim 13, wherein the nature-derived organic reducing agent comprisescitric acid and the relithiation solution comprises 0.01-4M LiOH and0.01-2M citric acid.
 17. The method of claim 14, wherein the lithiumsalt is selected from the group consisting of LiOH, Li₂SO₄, LiCl,LiC₂H₃O₂, and LiNO₃.
 18. The method of claim 13, wherein the sufficienttime is within a range of 1 hour to 18 hours.
 19. The method of claim13, wherein the temperature is 60-120° C. and the sufficient time is atleast 5 hours.
 20. The method of claim 13, wherein the sinteringtemperature is in the range of 400° C. to 800° C. and sintering isperformed for a sintering time in a range of 50 to 300 minutes.
 21. Themethod of claim 20, wherein the sintering time comprises temperatureramping to gradually heat the lithiated material at a controlled rate.22. The method of claim 13, wherein the relithiation solution isrecyclable and reusable for subsequent relithiation processes.